Recent data from studies of transgenic mice lend support to the mitochondrial theory of aging, which proposes that oxidative damage to mitochondrial DNA gives rise to mutations that accumulate with age, resulting in mitochondrial dysfunction and contributing to age-related disorders. In the human brain, we have found that oxidatively-induced somatic mitochondrial DNA (mtDNA) mutations accumulate with aging to reach high levels. Mitochondrial complex I dysfunction plays a key role in Parkinson's disease (PD), and indirect data implicates mtDNA mutations as the cause of this mitochondrial dysfunction. Yet, we and others have been unable to identify clearly pathogenic inherited mtDNA mutations in most PD patients. Together, these observations raise the possibility that the accumulation of somatic mtDNA mutations in the brain plays a key role in the pathogenesis of PD. However, little is known regarding the cell types in the brain that accumulate these mutations, or whether or not the accumulation of these mutations plays a role in mitochondrial dysfunction and neurodegeneration. We hypothesize that somatic mtDNA mutations accumulate with aging in single neurons and glia, that the levels of somatic mtDNA mutations in neurons, and possibly in astrocytes, are greater in PD compared to age-matched controls, and that these mutations contribute to mitochondrial complex I dysfunction. We will use laser capture microdissection (LCM) to isolate single neurons (with and without Lewy bodies), astrocytes, and microglia from the substantia nigra and control regions of young and old human control subjects, and from early and late stage PD patients. We have developed and validated a highly sensitive cloning-sequencing strategy that we will use to analyze these cells for levels and patterns of somatic mtDNA mutations, and will assess the relationship between these mutations and mitochondrial complex I dysfunction. PUBLIC HEALTH RELEVANCE: Parkinson's disease (PD) is a common age-related neurodegenerative disorder that leads to progressive disability. Oxidative stress and mitochondrial dysfunction play key roles in the pathogenesis of PD. The proposed studies of somatic mtDNA mutations in single cells in the human brain will provide insights into the relationship between aging, oxidative injury to mitochondrial DNA and mitochondrial dysfunction, and might help to identify new treatment strategies to delay or prevent the disability of PD. [unreadable] [unreadable]